[PATCH] Add man page for vfs_syncops

[Simo]

On Fri, 2013-08-09 at 07:13 -0700, Christoph Hellwig wrote:
> On Fri, Aug 09, 2013 at 07:42:46AM -0400, Ira Cooper wrote:
> > > I don't know a single Unix filesystems from the last 40 years where this
> > > is not true.  Why isn't this the default behaviour in Samba instead of
> > > having it in a module that very few people are going to use?
> > >
> > 
> > ZFS behaves correctly here.
> 
> I'm not sure what your defintion of correct is, but ZFS behaves just
> like I explained above (which for a local filesystem falls under correct
> for me).
> 
> Even the code implementing it looks similar to what you'd see for other
> filesystems - transactions just get greated and commited, but the log
> only gets forced out if a per-fs sync flag is set, in the zfs cases this
> looks like:
> 
> 	if (zfsvfs->z_os->os_sync == ZFS_SYNC_ALWAYS)
> 		zil_commit(zilog, 0);
> 
> (the above is for the sync=always property, which is equivalent to the
> sync mount option in Linux, or the wsync option in IRIX.
> 
> > And fsync only guarantees a file is correct,
> > not the filesystem.
> 
> fsync guaranteed the file behind the file descriptor is written back,
> which might also be a directory.  While Posix doesn't mention
> directories explicitly the fact that fsync is explicitly required to
> work on read-only file descriptors is an artefact of this that made it
> into the standard.
> 
> Calling fsync after directory operations is the most portable way to
> force directory operations to disk, and if you can actually find a
> filesystem where it's not required it should at least be harmless as
> there won't be anything left to sync on this fd.
> 
> > So, I 100% see the need for this.  And I'd want there to at least be a flag
> > to disable it.
> 
> If I wasn't clear enough - I agree with the need, but it should be the
> default and not hidden in an obscure VFS module.

+1

Simo.